Patent Literature 1, listed below, describes that a 5-(thiazol-4-yl)indolin-2-one derivative represented by the following formula (V):

wherein R1a, R2a, R3a, R4a, R5a, R6a, R7a, R8a, R9a, R10a, R11a, R12a, and R13a are a hydrogen atom, an alkyl group or a substituted alkyl group, an aryl group or a substituted aryl group, or the like, na is an integer selected from 2, 3, 4, 5, and 6, are useful for a pharmaceutical product.
Patent Literature 2, listed below, describes a production method of Sunitinib, an indolin-2-one derivative, with the purity thereof improved by reacting ethyl iodide with the compound that is de-ethylated at the terminal amino group of Sunitinib in the presence of a base to ethylate the terminal amino group.
Patent Literature 3, listed below, describes that impurities comprised in a crudely purified product of Sunitinib are reduced by adsorbing the de-ethylated compound of an intermediate for Sunitinib onto silica gel and separating by filtration.